Semipermeable membranes in the form of hollow fibers have been used to separate components in fluid mixtures for many years. Hollow fibers, which can be spun from a wide variety of materials which have suitable membrane separation properties, provide high surface area for contact with the fluid mixture from which it is desired to separate certain components, some of which will pass through the membrane material faster than other components. This enables the recovery of such faster permeating components or the enhancement of the purity of the fluid in a slower permeating component, or both.
Large numbers of hollow fibers of similar length are generally grouped together in a pressurizable shell or housing in which opposite ends of the fibers are potted and sealed in a material which serves to form a tubesheet at each end, similar in fashion to a shell and tube heat exchanger. The potting material is cut to open the bores of the fibers which pass through it. The volume within the shell which has access to the exteriors of the fibers (shell side) is effectively sealed by the tubesheets and other peripheral sealing devices from the volumes within the shell which are in open communication with the fiber bores.
Such devices can be used to separate liquid mixtures or to separate vapors or gases from liquids, but have found particular utility in the separation of gases, such as in air separation to purify nitrogen.
A number of methods for making bundles of hollow fibers suitable for fabrication of membrane modules are taught in the patent literature. For example, U.S. Pat. No. 3,228,877, Mahon (1966), describes the concept of using hollow fibers in a gas separating apparatus. In a subsequent development, U.S. 3,422,008, McLain (1969) discloses a method to wind hollow fibers spirally around a cylinder core. This permits the winding of hollow fibers to form a bundle shape such that the bundle is in an annular form with narrow flow channels which improve fluid flow distribution on the shell side of the fibers. A method to make a coreless annular array of helically wound fibers is described in U.S. Pat. No. 4,045,851 to Ashare, et al. (1977), and another method for making coreless hollow fiber membrane bundles is described in U.S. Pat. No. 4,351,092, Sebring, et al. (1982) wherein the fibers are interlaced with one another in left-hand and right-hand helices at angles to the common axis of rotation of the rotary members which form the fiber bundle. The foregoing disclosures describe modules which are typical and in which the fluid mixture to be separated is exposed to substantially all of the fibers at one time. Modules have been developed, however, in which the fiber bundles are partitioned into sections in order to manipulate the flow distribution of the feed material or of the permeate.
U.S. Pat. No. 4,676,808, Coplan (1987) describes a hollow fiber membrane module in which the potted ends of the fibers are cut differently at each end so that fibers opening at one end are closed at the other, and visa versa. Two concentric fiber bundle sections are thus formed in one module to simulate two modules arranged in series. The arrangement is said to form two permeates of different compositions since each bundle section is encountered by the feed gas as it flows radially from an outer cylindrical space to a central axial core from which it exits. The feed gas, therefore, flows perpendicularly to the flow of permeate gas within the fiber bores.
Russian patent SU256,132 (1987) discloses concentric baffling in a hollow fiber bundle of a separation module in order to channel feed mixtures back and forth through the shell side of the bundle. One permeate is taken from one end of the fiber bundle so that the feed flows concurrently with part of the permeate and countercurrently to part of it. Reversing the feed does not change this relationship. The separation module cannot function like two modules in series since only one permeate is produced.
U.S. Pat. No. 4,880,440, Perrin (1989) discloses hollow fiber membrane separation modules having two different types of fiber membrane possessing different fluid component separation characteristics for production of two permeate streams differing in composition and one raffinate stream from a feed mixture. The two types of fiber are wound helically on a mandrel, either intertwined or in alternating layers, but spaced so that only one of each fiber course extends to one end of the fiber bundle. When each end of the bundle is potted in a tubesheet and cut, only the type of fiber which extends to that end of the bundle is severed and opened for permeate to exit the bores of the fibers. Although feed is shown entering one end of the entire bundle on the shell side and raffinate leaving at the opposite end, the permeate streams necessarily flow in opposite directions. Consequently, countercurrent flow with the feed is achieved, if at all, only with respect to one of the permeate streams.
U.S. Pat. No. 4,929,259, Caskey, et al. (1990) discusses the advantage of countercurrent flow in hollow fiber membrane modules where the feed is to the bore side of the fibers. Concentric baffles, or a helical baffle, within the fiber bundle and on the outside toward the casing, are arranged to channel permeate flow countercurrently to the passage of feed fluid through the bores of the fibers. Sweep gas can be introduced through the core of the module to pass on the shell side of the fibers and enhance countercurrent flow of the permeate. No disclosure is included for operation of the module with feed to the shell side of the bundle, nor would reversing flow through the apparatus disclosed produce permeate streams differing in composition.
In hollow fiber membrane modules developed for air separation, feed gas channeling and deviations from countercurrent flow patterns can cause significantly lower performance. In modules operated with the feed through the bores of the fibers, uniformity of feed flow distribution and a good approach to countercurrent flow are easily achieved. A discussion of the effects of flow patterns is given by Antonson et al., "Analysis of Gas Separation by Permeation in Hollow Fibers", Ind. Eng. Chem. Process Des. Dev., Vol. 16, No. 4 pp 463-9(1977).
Hollow fiber membrane modules can in general be made by using either dense, asymmetric or thin film composite fibers. Brief descriptions of the three kinds of fibers are given below:
(a) Dense fibers have walls of uniform density and essentially zero porosity. They are usually made by melt spinning. PA1 (b) Asymmetric fibers have a thin dense skin (which constitutes the separating layer) imbedded in a wall with a gradation of porosity through its thickness. They are made by phase inversion processes. PA1 (c) Thin film composite fibers have a single or multiple coating of one or more polymers applied to the surface of a porous substrate fiber that provides a support for the coating(s).
Whereas dense and asymmetric fibers are usually made of a single polymeric material thin film composites are generally made by applying a coating different from the material of the substrate, this results in weaker adhesion between the separating layer and the substrate. In this case the probability of rupture is high when feed pressure is applied to the side opposite to the coating. Hence for thin film composite fibers coated on the exterior and for asymmetric membranes with the separating layer on the exterior shell-side feed is desirable.
Non-uniformity of feed flow distribution and deviations from countercurrent flow are valid concerns in modules adapted for shell-side feed. While many problems in gas separation efficiency can be appropriately addressed by simply using multiple modules connected in series, this is a costly solution because of the increased number of pressure vessels which are required.